Method of producing tantalum and other rare refractory metals by electrolysis of fused compounds



Patented Jul 21', 1931 UNITED STATES PATENT OFFICE FRANK H. DRIGGS, OF BLOOMFIELD, NEW JERSEY, ASSIGNOR TO WESTINGHOUSE LAMP COMPANY, A CORPORATION PENNSYLVANIA METHOD OF PRODUCING TANTALUM AND OTHER RARE REFRACTORY METALS BY ELEGTROLYSIS OF FUSED COMPOUNDS No Drawing. Application filed May 4,

This invention relates to the art of metalart of preparing the so-called rare refractory metals comprising those metals of the tantalum, tungsten, manganese, zirconium and thorlum groups and comprises essentially a method of preparing these metals by the electrolysis of their fused salts.

The process is directed specifically to the production of tantalum in a powder form by the electrolytic decomposition of a .dou-

,ble alkaline tantalum fluoride compound which is in solution in an alkaline fluoride fusion comprised of potassium or sodium fluorides or a mixture of the two alkaline fluorides to which has been added a proportion of a compound of tantalum which is soluble in the fusion mixture to yield an ionized component thereby which substantially has a lower anode decomposition voltage than the fluorine of the bath. Such a suitable tantalum compound is tantalum voxide (Ta O but other oxygen carrying compounds or other compounds which essentially produce the same effect may be used in the practice of my invention.

Heretofore in the-prior art many methods have been devised for the preparation of the rare refractory metals. Due to the extremely high melting points of these refractory metals, most of these processes produce these metals in the powder form which must subsequently be compacted and sintered to a coherent ingot. In the powdered form these metal powders are extremely active and react at elevated temperature with gases and volatile'metalloidal impurities to form compounds which are extremely stable and detrimental to subsequent mechanical working. Most of the methods disclosed for the production of rare refractory metals are expensive as regards the reagents employed in the reduction reaction and nearly all of them yield as a final product an impure metal 'powder which must be subsequently purified.

1928. Serial No. 275,264.

is to provide a cheap and eflicicnt method of producing rare refractory metals.

Another object of this invention is to provide a simple and efficient electrolytic method of producing rare refractory metals.

Another object of this invention is to provide a means of overcoming the so-called anode effect usually experienced in electrolytic decompositions of fused baths so that such a process may be rendered thereby simple and eflicient to operate.

Another object of this invention is to provide a method of preparing tantalum powder by electrolysis which is of such a high degree of purity that it may subsequently be compacted into bars, heat-treated in vacuum to a coherent state and thereafter mechanically deformed to desired shapes, such as rods, wire, filaments, sheet and the like.

Other objects will be apparent as the process is disclosed.

In accordance with my invention I pre-,

pare a fusion mixture comprised of potassium and sodium fluorides alone or admixed in any desirable proportions which fluorides are preferably free of impurities that are lower than tantalum or the rare refractory metal to be prepared in the electro-motive series of elements, such as iron, chromium, nickel, copper, etc., as itis apparent that such impuritics will be deposited from the bath before the refractory metal. In practice, therefore,

prior to the actual preparation of the fusion mixture I preferably make certain that these alkali metal fluorides are completely free of such contaminating impurities.

Other halogen fusion mixtures may be employcd, such as fusible mixtures of double chlorid'es, etc., but'fo-r my purpose I have found that the fluorides are the most satisfactory material to employ in the practice of my process;

'I also prepare as an integral part of my process a very pure grade of a double halogen compound of the rare refractory metal such as the double fluoride compound of the rare refractory metal; typical of the double flourides are the tantalum-potassium double fluoride having the formula K TaF orthe double therewith, notably columbium. After recrystallizing this double potassiumtantalum compound, for instance, several times to effect substantial purification, it may be dried at 100 C. for several hours in the air to eflectdehydration without effecting substantial decomposition or altering of its fundamental structure and it may be kept indefinitely without decomposition.

I also prepare as an integral part of my process a very pure grade of the oxide of the rare refractory metal such as tantalum oxide or any other oxygen carrying compound which is suitable in' my process, such as the o'xy-fluoride, which is preferably made from portions of the double fluoride compound as previously prepared or' by any other means as desired. A hi h degree of purity is desired in this pro uct also as the successful preparation of pure metal is primarily dependent upon the use of materials not contaminating with respect to the metal.

I next provide a suitable apparatus and receptacle in which to conduct the electrolysis which for my purpose may be composed of graphite, car on, or may be composed of any material resistant to the corrosive action of the fused flux. Within this receptacle I provide two electrodes suitably spaced, depending upon the size and depth of the container and the flux which is being electrolyzed.

I have determined that in the preparation of the rare refractory metals, I may substantially overcome the so-called anode effect which normally is encountered in the V electrolysis of fused fluorides of these and other metals in a certain novel manner which comprises essentially in the addition to the bath of a suitable compound of the rare refractory metal being deposited which is substantially soluble in the fusion mixture, to

v such an extent at least as to provide a means of obtaining in the fused bath an ionized constituent which may be more readily liberated at the anode than the fluorine content such as has heretofore been attempted. With tantalum the tantalum oxide (Ta O has been 'found to yield the best results and I therefore specify the use of this compound in preference to other known compounds which could be applied as it is most readily prepared in a relatively pure condition.

I have also determined that there must also be present in the electrolyte a compound of the rare refractory metal, such as tantalum, in which the tantalum componentinsolution in the fusion or on fusion becomes ionized as free positively charged metal ions,

and not as complex metal ions wherein the rare refractory metal is a component of an acid or negatively charged radical.

halogen compounds. The double fluoride of the rare refractory metal with the alkali or alkaline earth metals is preferred as these compounds are the most stable of the double halogens and are the most easily and readily prepared inthe pure state.

By observing these conditions I may obtain from a fused fluoride fusion deposition of the/refractory metal, such as tantalum, in a crystalline powder form at the cathode and a liberation of oxygen from the anode, and I may make the electrolytic process a .continuous one by continued additions of the soluble oxygen carrying component to the bath during the progress of the run, said additions at no time exceeding in amount the solubility limit of the bath for the oxide material. f

Furthermore, by this method of production I am enabled to produce the so-called rare refractory metal powders in a cheap and efficient continuous manner and through the judicious use of the materials employed I can produce these metals in the powdered. form in a high state of purity.

As a specific embodiment of my invention This compound may be any of the stable double I will describe the production of tantalum 'C. the amounts of KF to NaF should be kept to nearly equal proportions by weight. To this fusion mixture may be added up to fifty (50) percent by weight of the purified double tantalum-potassium fluoride, and the entire mixture brought to a quiet fusion in a graphite crucible. I prefer to use in fusing this material, external heating suchas by surrounding the crucible with a wire wound resistance type furnace, as duringthe subsequent electrolysis of the bath the resistance of the bath is insuflicient to maintain the temperature of the fusion.

Thereafter to the fused bath I add approximately ten (10) percent (by weight) tantualum oxide (Ta O that is as compared to the amount of, the double fluoride compound previously incorporated in the fusion. When this oxide is completely dissolved in the fusion I then pass a direct electric current -therethrough regulating the voltage to give the passage of the electric current through the bath will vary, of course, with the distance between the electrodes and somewhat with the composition of the fused bath.

By maintaining the bath at approximately 70( C. the electrolysis will proceed in a quiet manner until the metal component equivalent to the amount of the oxygen carrier component of the bath has been com pletely deposited out. at the cathode. Frequent additions of small increments of tantalum oxide to the bath during the electrolysis will keep the deposition of the tantalum going indefinitely and the electro-chemical characteristics of the bath thereby will remain practically constant throughout the entire electrolysis period." Said additions of oxide to the bath, however, should not exceed the solubility limit of the'bath for the oxide, it being appreciated that in addition to deleteriously effecting the melting point of the bath, the excess of oxide would serve no useful purpose in the operating ;of the bath. I have determined that the solubility of the oxide is dependent upon the proportions or amount of the double fluoride in the bath and for some purposes it may be necessary to increase the amount of the double fluoride over that specified by me in order to increase the amount of-available ionized oxygen in the bath.

The conclusion of the electrolysis may be detected by a sharp rise in the voltage required to force the current through the bath developing what is known as an anode effect which indicates that the oxygen carrier has been all exhausted and the fluorine content is being liberatedinstead. The anode is thereupon withdrawn from the bath and fusion mixture allowed to cool in position.

The cooled crushed fused mass is then lixiviated in any suitable manner to remove the water soluble alkali fluorides and the undecomposed double alkaline tantalum fluoride residues. These may subsequently be re- 7 The remaining insoluble material isput through a gravity separation process to concentrate the metal particles after which it is thoroughly washed with nitric acid to remove acid soluble impurities. Following which the metal powder is washed free of moisture with alcohol followed by an ether wash. After drying in vacuo the metal powder is substantially ready,for compacting, sintering and treat ing operations to produce a coherent ductile body. The procedure employed in obtaining a coherent ductile body of tantalum may be substantially as described in copending application Serial No. 717,490,'fi1ed June 5, 1924, entitled ductile thorium and the method of making the same and assigned to the same Qassignee as the present invention or it may be accomplished by any other suitable procedure which will essentially observe the precautions of thorough evacuation of absorbed fluoride fusion.

or adsorbed gases from the metal body prior to actual sintering thereof. I

By observing extreme care in the purity of the components of the fusion mixture which is being electrolyzed, such as the alkali fluorides, double fluorides of the rare refractory metal, and in the ionizable component of the bath, a high degree of'purity and freedom of deleterious intermetallic impurities such as iron nickel, silicon, etc. will be obtained; It q will not be necessary therefore, to substantially fuse the metal body and to substantially vaporize or boil'out such impurities as heretofore encountered in prior processes developed. With proper heat treatment in "the fusion mixture as it is obvious that many variations in the process will be apparent to one skilled in the art, and such variations are anticipated which will fall within the claims as set forth. l/Vhat is claimed is: o

1. The process of producing rare refrac-' tory metals which comprises electrolytically. depositing the rare refractory metal from a fusion mixture composed of alkali metal i halides, a proportion of a double alkali rare refractory metal halide compound and a proportion of an ionizable oxygen containing compound of the rare refractory metal which is soluble in the halide fusion mixture.

2. The process of producing rare ref-ractory metals which comprises electrolytically depositing the rare refractory metal from a fusion mixture composed of alkali metal fluorides, a proportion of a double alkali rare refractory metal fluorides compound and a proportion of an oxygen containing compound of the refractory metal which is soluble at least in part in the bath.

3. The process of producing rare refractory metals which comprises electrolytically dissociating a fluoride fusion containing alkali fluorides, rare refractory metal double Ell) fluoride compounds, and a proportion of an ionizable oxygen carrying'compound of the rare refractory metal which is soluble in the fluoride fusion mixture.

I 4. The process of preparing tantalum which comprises electrolytically decomposing a fused double fluoride compound of tantalum in solution in a fusion mixture comprised of alkali metal halide compounds in the presence of an oxygen carrying compound of tantalum which is soluble in the 5. The process of preparing tantalum which comprises electrolytically dissociating a fluoride fusion mixture comprised of alkali metal fluorides containing a proportion of double potassium-tantalum fluoride and a smaller proportion of tantalum oxide.

6. The process of preparing tantalum which comprises electrolytically decomposing a fusion mixture comprised of potassiumtantalum fluoride (approximately 1 part) alkaline fluorides (2 parts) and tantalum oxide (approximately 10 percent of the weight of the tantalum double fluoride compond).

7. A fusion mixture suitable for the production of rare refractory metals by electrolytic decomposition comprisin in part of alkali metal fluoride compounds, a proportion of a double alkaline'fluoride compound of the rare refractory metal to be deposited, and a proportion of an oxygen containing compound of the rare refractory metal which is soluble and ionizable in the fluoride fusion.

8. A fusion mixture suitable for the production of tantalum metal powder comprising alkali metal fluorides containing a proportion of double potassiumrtantalum fluoride compound, and a smaller proportion of an oxygen carrying compound of tantalum which is soluble in and ionizable in the fused fluoride fusion.

In testimony whereof, I have hereunto subscribed my name this 2nd day of May, 1928.

' FRANK H. BRIGGS. 

